This invention is directed to a process for separating pentafluoroethane from a mixture of pentafluoroethane (HFC-125) and chloropentafluoroethane (CFC-115) by extractive distillation.
New regulations have been designed to protect the ozone layer from possible damage by fully halogenated chlorofluorocarbons. Pentafluoroethane (HFC-125) is a valuable non-chlorine containing fluorocarbon that is especially useful as a refrigerant, blowing agent, propellant, fire extinguishing agent or sterilant carrier gas. Pentafluoroethane is usually prepared by chlorofluorinating perclene to produce a mixture of 1,1,2-trichlorotrifluoroethane (CFC-113), 1,2-dichlorotetrafluoroethane (CFC-114) and 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123). After removal of 1,1,2-trichlorotrifluoroethane, the remaining mixture can be fluorinated by various processes, resulting in a mixture containing pentafluoroethane (HFC-125) and chloropentafluoroethane (CFC-115), and smaller amounts of other fluorinated compounds, e.g., hexafluoroethane (FC-116). Various other methods for making pentafluoroethane together with chloropentafluoroethane are known and the resulting mixture made by other processes can likewise be treated by the extractive distillation process of the present invention to recover pentafluoroethane. Unfortunately, the mixture of pentafluoroethane and chloropentafluoroethane form a near-azeotrope. The boiling points of the halogenated hydrocarbons are very close, -48.5.degree. C. for pentafluoroethane and -38.7.degree. C. for chloropentafluoroethane, and their relative volatility is below 1.1 at concentrations of pentafluoroethane greater than 87.5 mole %, and below 1.01 at concentrations above 95 mole %. The boiling point and relative volatilities indicate that it would be extremely difficult, if not impossible, to recover substantially pure pentafluoroethane from such mixtures by simple distillation and, therefore, extractive distillation is an alternative procedure that could possibly be used. However, the main problem employing an extractive distillation process is discovering an extraction agent which will sufficiently aid the desired separation process to make up for the need to add a new separation step to remove and recycle the extractive agent. Methods which have been used to predict what extractive agents are likely to work have been described by L. Berg in Chem. Eng. Progress, Vol. 65, No. 9, pages 52-57, September 1969. In discussing extractive distillation, it is stated in the article that "hydrogen bonds appear to be an important factor since all successful extractive distillation agents are highly hydrogen bonded liquids . . . Thus, the criteria for successful extractive agents are that they boil considerably higher than the compounds being separated, form no minimum azeotropes with the components, and be a highly hydrogen bonded liquid, that is, Class I or Class II of the hydrogen bond classification. Phenols, aromatic amines (aniline and its derivatives), higher alcohols, glycols, etc. are examples of successful extractive agents." The present invention makes possible separation of pentafluoroethane from chloropentafluoroethane by extractive distillation with compounds that however, are not highly hydrogen bonded.